Semiconductor lasers are formed of multiple layers of semiconductor materials. The conventional semiconductor diode laser typically includes an n-type layer, a p-type layer and an undoped layered active structure between them such that when the diode is forward biased electrons and holes recombine within the active structure with the resulting emission of light. The layers adjacent to the active structure typically have a lower index of refraction than the active structure and form cladding layers that confine the emitted light to the active structure and sometimes to adjacent layers. Semiconductor lasers may be constructed to be either edge emitting or surface emitting.
A semiconductor laser that emits photons as electrons from within a given energy band cascade down from one energy level to another, rather than emitting photons from the recombination of electrons and holes, has been reported by a group at AT&T Bell Laboratories. See, J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, Science, Vol. 264, pp. 553, et seq., 1994. This device, referred to as a quantum cascade laser (QCL), is the first reported implementation of an intersubband semiconductor laser. The basic light-generation mechanism for this device involves the use of 25 active regions composed of 3 quantum wells each. Injection by resonant tunneling occurs in the energy level (level 3) of the first, narrow quantum well. A radiative transition occurs from level 3, in the first well, to level 2, the upper state of the doublet made by two coupled quantum wells. Quick phonon-assisted relaxation from level 2 to 1 insures that level 2 is depleted so that population inversion between levels 3 and 2 can be maintained. Electrons from level 1 then tunnel through the passive region between active regions, which is designed such that, under bias, it allows such tunneling to act as injection into the next active region.
Lasing for such devices has been reported at 4.6 .mu.m up to 125K with threshold-current densities in the 5 to 10 kA/cm.sup.2 range. F. Capasso, J. Faist, D. L. Sivco, C. Sirtori, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, Conf. Dig. 14th IEEE International Semiconductor Laser Conference, pp. 71-72, Maui, Hi. (Sep. 19-23, 1994). While achieving intersubband lasing in the mid- to far-infrared region, the thresholds were two orders of magnitude higher than "state-of-the-art" practical diode lasers. The reason for the high thresholds is that the transition from level 3 to 2 is primarily nonradiative. The radiative transition, with momentum conservation, has a lifetime, T.sub.R, of about 26 ns, mostly due to the fact that it involves tunneling through the barrier between the first and second quantum well. By contrast, the phonon-assisted transition, T.sub.32, has a relatively short lifetime, i.e., T.sub.32 .apprxeq.4.3 ps. As a result, phonon-assisted transitions are about 6000 times more probable than photon-assisted transitions; that is, the radiative efficiency is 1.6.times.10.sup.-4, which explains the rather high thresholds.
Faist, et al. proceeded to improve their QCL device by making two-well active regions with a vertical transition in the first well, and providing a multi-quantum barrier (MQB) electron reflector/transmitter (mirror). J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, and A. Y. Cho, Appl. Phys. Lett., 66, 538, (1995). As a result, the electron confinement to level 3 improved (i.e., the reflection aspect of the MQB mirror suppresses electron escape to the continuum), and threshold current densities, J.sub.th, as low as 1.7 kA/cm.sup.2 at 10K were achieved. However, the basic limitation, low radiative efficiency (.about.1.6.times.10.sup.-4), was not improved, since phonons still dominate the level 3 to level 2 transition. Using a 2 QW active region with a vertical transition in the first well, J.sub.th values as low as 6 kA/cm.sup.2 at 220K were obtained. J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, "Continuous wave quantum cascade lasers in the 4-10 .mu.m wavelength region," SPIE vol. 2682, San Jose, pp. 198-204, 1996. Recently, an improved version of the vertical transition design has been operated pulsed at 300K, the first mid-IR laser to operate at room temperature in the 5 .mu.m wavelength regime. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, "Room temperature mid-infrared quantum cascade lasers," Electron. Lett., vol. 32 pp. 560-561, 1996. Despite this rapid improvement in the performance capabilities of GaInAs/InP-based QC lasers, it is unlikely that they will ever be able to operate continuous wave (cw) at 300K due primarily to the fact that their radiative efficiency is inherently poor. This poor efficiency is quantified by noting that the non-radiative LO-phonon-assisted relaxation time for the upper laser states is about 1.8 ps and the radiative relaxation time is 4.2 ns. Consequently, the reported QCL laser devices are fundamentally inefficient, and thus impractical, due to the dominance of phonon-assisted transitions.